In the related art, an occupant detection apparatus of a vehicle may detect a seat load by using a strain sensor provided between a seat frame and a slide rail, as described in, for example, JP 2011-16424A (Reference 1). Further, for example, as described in JP 2006-242907A (Reference 2), an electrostatic capacitance sensor in which a sensor electrode is disposed inside a seat skin is used. Further, for example, in an occupant detection apparatus described in U.S. Pat. No. 7,059,446 (Reference 3), a bladder (bag body) filled with a liquid (gel) is disposed below a seat cushion. Then, it is configured to detect the seated state of the occupant on the seat, based on the internal pressure change of the bag body.
However, in a technique in the related art described in Reference 1, for example, a strain sensor provided in the frame of a seat is likely to be affected by a shape change or a position change occurring in the installation location thereof, when a seat operation such as sliding or reclining a seat is performed, or a collision occurs. An error caused by this may hamper the improvement of detection accuracy. A technique in the related art described in Reference 2 has a structure in which a sensor electrode of an electrostatic capacitance sensor radiates electric force lines in the seating surface direction of a seat. Therefore, for example, there is a problem that it is likely to be affected by disturbance of which a noise source is a liquid such as a beverage or an electronic device (such as a personal computer) placed on a seat. In a configuration for detecting an internal pressure of a bladder filled with liquid as a technique in the related art described in Reference 3, the liquid in the bladder is likely to be affected by a temperature change, and thus there is a problem that it is difficult to secure stability of load detection thereof.
Thus, a need exists for an occupant detection apparatus which is not susceptible to the drawback mentioned above.